> > >> -----Original Message----- >> From: Peter Coghlan via cctalk <[email protected]> >> Sent: 19 October 2025 18:32 >> To: General Discussion: On-Topic and Off-Topic Posts <[email protected]> >> Cc: Peter Coghlan <[email protected]> >> Subject: [cctalk] Re: Rainbow H7842 PSU >> >> >> > >> >> -----Original Message----- >> >> From: Rob Jarratt <[email protected]> >> >> Sent: 19 October 2025 14:04 >> >> To: [email protected]; 'General Discussion: On-Topic and Off-Topic Posts' >> >> <[email protected]> >> >> Subject: RE: [cctalk] Re: Rainbow H7842 PSU >> >> >> >> >> >> >> >> > -----Original Message----- >> >> > From: Rob Jarratt via cctalk <[email protected]> >> >> > Sent: 14 October 2025 21:42 >> >> > To: 'General Discussion: On-Topic and Off-Topic Posts' >> >> > <[email protected]> >> >> > Cc: Rob Jarratt <[email protected]> >> >> > Subject: [cctalk] Re: Rainbow H7842 PSU >> >> > >> >> > >> >> > >> >> > Oh my! I have clearly got my understanding the wrong way around, >> >> > not sure how I did that because I read the datasheet carefully. >> >> > Somehow, I got confused. I re-measured and found 1IN+=5.5V, >> >> > 1IN-=9.4V, Power OK=6.7V, but GND (on the 393) is -13V. So as you >> >> > say Power OK should be -13V and AC OK H would be asserted. I guess >> >> > this must mean that the comparator itself is faulty. I have some >> >> > 393s, so I will replace it and see what >> >> happens. >> >> > >> >> >> >> Today I replaced the LM393P with a new LM393N. The AC OK H signal is >> >> now working correctly. However, the outputs are a bit on the low >> >> side. The +5V output is 4.37V, the 6% tolerance quoted in the >> >> technical manual would allow a minimum of 4.7V. The +12V output is >> >> 11.3V, 6% tolerance would allow a minimum of 11.3, so only just OK. >> >> Any suggestions on what I could look at here, maybe the output >> >> capacitors? I checked the two capacitors on the +5V output and the >> >> ESR seems OK when measured in circuit, but perhaps the big one before >> >> the smoothing choke should be replaced? NB Raw 5V is about 4.2V >> > >> > Thinking about this a bit more, I am thinking the 5V output capacitors >> > because the 5V output is proportionally lower than the 12V output. But >> > they are both too low and I guess that implicates the drive control. I >> > seem to remember replacing the PWM fairly recently so I would hope its >> > not that, but maybe one of its inputs. Vstart is only 11V, which seems a >> > bit >> suspicious. >> > Either the 7812 is bad or something is pulling it down. But it looks >> > like I also replaced the 7812. I might try using a bench PSU to >> > provide power to the 7812 to see how it behaves. >> > >> >> I still think the first thing to be done is to figure out whether Vz is >> correct or >> not, ideally by reading the markings on the zener diode or otherwise figuring >> out what voltage is supposed to be across it and then measuring the voltage >> directly across it to confirm or deny that it is correct. > > I measured Vz at 4.3V. In my notes I have previously measured 5V or 5.4V, > possibly on different H7842 PSUs (my notes are not clear). I have found the > Zener diode, it is hard to make out the markings without lifting it, but I > *think* the markings are > > 00 > 426 > > Not sure if this means the Zener voltage is supposed to be 6V? I will lift > it when I get a bit of time, probably in a couple of days, so I can look at > it properly and check it out of circuit with my DMM. >
Could it be a 1N4626 ? According to the datasheet, this would be a 5.6V Zener which seems plausable. A 4.3V unit would be a 1N4623 according to the same datasheet. 4.3V seems like an odd value though. You mentioned measuring Vz at 6.4V in a previous mail in this thread so there is a possibility the voltage might be varying which would not be good news. Regards, Peter. > > Thanks > > Rob > >> >> The most likely causes of only 11V coming out of the 7812 are either the >> input >> to it is low (it needs to be at least several volts higher than 12V) or the >> current >> drawn from it is excessive, in which case it will be hot or it is >> faulty. There is also a remote possibility that is oscillating due to a bad >> decoupling capacitor. >> >> (It is possible that nothing cares that Vstart is a bit low, some of the >> comparator inputs come from it via voltage dividers but the more critical >> ones >> seem to be referenced to Vz.) >> >> There is also the odd 0.7V difference between two ground connections in >> different parts of the power supply that needs to be accounted for or fixed >> if >> it turns out to be a fault. >> >> Once these items are dealt with, the low outputs can be addressed if they are >> still low after dealing with the other issues. >> >> Regards, >> Peter Coghlan.
